Variable inductance device



Jan.. 6, 1942. R. M. DAUGHERTY 2,268,742

VARIABLE INDUCTANCE DEVICE Filed Nov. 4, 1940 ual/1.

ROGER N- DAUGHERTY INVENTOR.

BYM'

ATTORNEY.

Patented Jan. 6, 1942 VARIABLE INDUCTANCE DEVICE Roger M. Daugherty, Cincinnati, Ohio, assignor to The Crosley Corporation,

corporation of Ohio Cincinnati, Ohio, a

Application November 4, 1940, Serial No. 364,161

Claims.

This invention relates to electrical devices designed for use with tunable electrical circuits and is particularly concerned with variable inductance devices of the type having compressed comminuted magnetic cores arranged to be movable relatively to the coils with which they are associated. y

Devices of this kind are well known and may consist of cores of finely divided iron each movable longitudinally within a coil wound on a tube of dielectric material and connected into the circuit desired to be tuned. The relative motion of the coil and core produces changes in inductance which permit tuning the resonant circuit of which the inductance forms a part. Tuning in this manner is known as permeability tuning.

Such devices must be very accurately made in order that the clearance between the relatively movable members be as small as possible yet at the same time maintaining suilicient clearance to permit the free movement of one member inside the other. During use such devices are often near or adjacent the loud speaker of the radio receiver, and the sound waves emanating therefrom tend to set up vibrations in the core-coil assembly which are likely to cause the members to move laterally with respect to one another, even though they have been reasonably accurately iltted together. This results in mistuning the circuitsin which the variable inductance devices are included, resulting, in turn, in an undesired modulation of the radio waves at the frequency or frequencies at which vibration occurs. This phenomenon is called microphonic action or acoustical feed-back and manifests itself in a disagreeable howl in the speaker,

It is an object of the present invention to yeliminate this difficulty and to provide a permeability tuned device which is not affected by the microphonic action of the speaker. It is a further object to provide a permeability tuned device which is simple of construction, easy of operation and eiilcient in result,

The novel features that I consider characteristic of my invention are set forth in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments, when readin connection with the accompanying drawing, in which:

Figure 1 is a sectional view of an illustrative magnetic core associated with a coil within which it is longitudinally movable for varying the inductance of a circuit;

Figure 2 is an enlarged fragmentary section showing a core constructed in'accordance with the invention.

I have found that it is possible to eliminate acoustical feedback in inductively tuned circuits of the nature described by spraying or otherwise applying a coating of a binder to a magnetic core and afterwards spraying upon the tacky binder fllm a fiocculent material such a finely divided rayon, cotton, wool or other fibrous material. When the binder or base has dried the flocks adhere firmly thereto. The core may then be inserted in the coil at which time the flocks lie down in a direction generally parallel to the axis of the core yet they have a tendency to stand up somewhat thereby acting as a cushion completely filling the space between the relatively movable core and coil. This substantially eliminates any relative movement between the two members, particularly in a direction at right angles to the axis of the core.

Ground mica and similar compositions have -previously been applied to iron cores in order to permit their ready longitudinal movement relative to their respective coil forms. Due to their slippery or soapy characteristic this type of material has a lubricating effect and tends to reduce noises due to friction between the magnetic core and the coil form. However, such treatment does not eliminate noises due to detuning in turn caused by the effect of sound waves emanating from the speaker. It may be desirable in some cases to apply to a core to which I have applied a vflocculent coating in accordance with the present invention an additional .coating of ground mica in order to ensure the elimination of the static charges due to friction.

Referring to the drawing in which I have illustrated one way in which the invention may be carried out, Figure 1 shows a magnetic core i0 of comminuted metal particles of a type well known in the art, adapted and arranged to be moved longitudinally of and within a closely fitting tube ii, which may be a phenolic condensation product. Around the tube i I is wound a conductor I2. The diameter of the tube, the size of Wire and the number and position of the turns are such as to cooperate with the core to give the required range of inductance values as the core is moved between its limiting positions within the magnetic field of the coil when the same is energized by alternating current.

As shown in the enlarged fragmentary view of Figure 2, I apply to the core I a coating of lacquer I3 of any suitable type and while the lacquer is still tacky I apply, as through a spray gun or blower, a layer of occulent material i4 such as finely divided rayon bers. After the lacquer has dried, which in the case of a 'quick-drying lacquer will be within a few hours, the core is ready to be slipped into its related coil-carrying tube, but I may apply to the occulent'coating an outer layer (not shown) of ground mica.

The core thus constructed slips easily 4in and out of its respective tubular housing H, the

flocks tending to lie down as the core moves within an annular aperture l5, as shown in the enlarged fragmentary view of Figure 2. The ocks, which form a plush-like surface. tend to straighten up, as in a pile fabric, thus filling the annular aperture and maintaining the core and coil in substantially xed lateral relationship. The ocks nevertheless permit relative longitudinal movement between the members l@ and Ii. Acoustic feedback is substantially entirely eliminated without any sacrifice to the ease by which the members can be moved relative to one another and Without the necessity of maintaining exceptionaltting precision during manufacture. Various modications of the inventionl will be apparent to persons skilled in the art. Instead of brous material of the fiocculent type, an equivalent material having a cushioning effect and lacking the ability to generate static electricity may be used. Again it may not be necessary to cover the core completely with iiock as the benefits of the invention may be obtained by coating a portion of the core, for instance between 1/3 andv M2 its length, which will eect suiicient cushioning to eliminate undesirable effects of play between the relatively movable members.

Having thus described my invention, I claim:

1. A device for tuning a circuit, comprising, a magnetic core, a coil concentric therewith and disposed about the core, the core and coil being adapted and arranged for relative movement longitudinally of one another, and a layer ofocks disposed between the core and coil, said flocks forming a plush-like surface.

2. A device for tuning a radio circuit, com-l prising, a magnetic core, a coil concentric therewith and disposed about the core, the core and coil being adapted and arranged for relative movement longitudinally of one another, a binder on said core and fiocculent material secured in said binder.

3. A device for tuning a radio circuit comprising, a core, a coil surrounding the same and concentric with the axis of the core, said core being adapted and arranged to move longitudinally of the coil to vary the inductance of the coil, and a occulent layer between the coil and core, said layer forming a plush-like surface.

4. A device for tuning a radio circuit comprising, a core, a coil surrounding the same and concentric with the axis of the core, said core being adapted and arranged to move longitudinally of the coil to vary the inductance of the coil, a binding layer disposed upon said core and a layer of occulent material secured in said binder.

5. A device for tuning a radio circuit comprising, a core, a coil surrounding the same and concentric with the axis of the core, said coil and core being adapted and arranged'to move longitudinally relative toone another to vary the inductance of the coil, a layer of lacquer on said core and flocks disposed on said lacquer layer and secured therein.

ROGER M. DAUGHERTY. 

